J.D. van Wees

Dynamics of intra-plate compressional deformation: the Alpine foreland and other examples

Abstract Intra-plate compressional structures, such as inverted extensional basins and upthrusted basement blocks, play an important role in the tectonic framework of the European Alpine foreland. Similar structures are observed on many continental cratons but occur also in oceanic basins and more rarely along passive continental margins. The World Stress Map shows that horizontal compressional stresses can be transmitted over great distances through continental and oceanic lithosphere. Although a number of geodynamic processes contribute to the build-up of intra-plate horizontal compressional stresses, forces related to collisional plate interaction appear to be responsible for the most important intra-plate compressional deformations. Such deformations can involve whole-lithosphere buckling and folding, crustal folding and, by reactivation of pre-existing crustal discontinuities, upthrusting of basement blocks and inversion of tensional hanging-wall basins. Mechanical aspects of basin inversion depend on the interplay of stresses and rheology of the lithosphere. Pre-existing crustal discontinuities weaken the lithosphere and play a crucial role in localizing intra-plate compressional deformations. Reactivation of relatively steeply dipping normal faults occurs when the angle between their strike and the compressional stress trajectory is smaller than 45°. Compressional deformations restricted to crustal levels involve ‘simple-shear’-type detachment of the crust at the level of the rheologically weak lower crust from the mantle-lithosphere; whole-lithospheric ‘pure-shear’-type compressional deformation is indicated for certain inverted basins. A distinction must be made between collision-related and anorogenic compressional/transpressional intra-plate deformations. The hypothesis is advanced that the stratigraphic record of collision-related intra-plate compressional deformations can contribute to the dating of orogenic events affecting the margin of the respective craton.

Mechanical controls on collision related compressional intraplate deformation.

Intraplate compressional features, such as inverted extensional basins, upthrust basement blocks and whole lithospheric folds, play an important role in the structural framework of many cratons. Although compressional intraplate deformation can occur in a number of dynamic settings, stresses related to collisional plate coupling appear to be responsible for the development of the most important compressional intraplate structures. These can occur at distances of up to ±1600 km from a collision front, both in the fore-arc (foreland) and back-arc (hinterland) positions with respect to the subduction system controlling the evolution of the corresponding orogen. Back-arc compression associated with island arcs and Andean-type orogens occurs during periods of increased convergence rates between the subducting and overriding plates. For the build-up of intraplate compressional stresses in fore-arc and foreland domains, four collision-related scenarios are envisaged: (1) during the initiation of a subduction zone along a passive margin or within an oceanic basin; (2) during subduction impediment caused by the arrival of more buoyant crust, such as an oceanic plateau or a microcontinent at a subduction zone; (3) during the initial collision of an orogenic wedge with a passive margin, depending on the lithospheric and crustal configuration of the latter, the presence or absence of a thick passive margin sedimentary prism, and convergence rates and directions; (4) during post-collisional over-thickening and uplift of an orogenic wedge. The build-up of collision-related compressional intraplate stresses is indicative for mechanical coupling between an orogenic wedge and its fore- and/or hinterland. Crustal-scale intraplate deformation reflects mechanical coupling at crustal levels whereas lithosphere-scale deformation indicates mechanical coupling at the level of the mantle-lithosphere, probably in response to collisional lithospheric over-thickening of the orogen, slab detachment and the development of a mantle back-stop. The intensity of collisional coupling between an orogen and its fore- and hinterland is temporally and spatially variable. This can be a function of oblique collision. However, the build-up of high pore fluid pressures in subducted sediments may also account for mechanical decoupling of an orogen and its fore- and/or hinterland. Processes governing mechanical coupling/decoupling of orogens and fore- and hinterlands are still poorly understood and require further research. Localization of collision-related compressional intraplate deformations is controlled by spatial and temporal strength variations of the lithosphere in which the thermal regime, the crustal thickness, the pattern of pre-existing crustal and mantle discontinuities, as well as sedimentary loads and their thermal blanketing effect play an important role. The stratigraphic record of collision-related intraplate compressional deformation can contribute to dating of orogenic activity affecting the respective plate margin.

On the origin of the southern Permian Basin, Central Europe.

Abstract A detailed study of the structural and stratigraphic evolution of the Southern Permian Basin during latest Carboniferous to Early Jurassic times, supported by quantitative subsidence analyses and forward basin modelling for 25 wells, leads us to modify the conventional model for the Rotliegend–Zechstein development of this basin. The Late Permian–Early Jurassic tectonic subsidence curves are typical for a Permian to Early Triassic extensional stage that is followed by thermal subsidence. However, a purely extensional model is extremely problematic because active faulting during this time is ‘minor’ and generally hard to document. Using inverse techniques to model the subsidence curves, we quantitatively show that a significant component of Late Permian and Triassic tectonic subsidence can be explained by thermal relaxation of Early Permian lithospheric thinning, and by delayed infilling of paleo-topographic depressions that developed during the Early Permian. In this interpretation, Stephanian–Autunian wrenching resulted in thermal destabilisation of the lithosphere, deep fracturing of the crust, disruption and erosion of its sedimentary cover and regional uplift of the area of the future Southern Permian Basin. Upon termination of wrench tectonics and associated volcanism, towards the end of the Autunian, the Southern Permian Basin began to subside in response to thermal contraction of the lithosphere. The evolving basin was isolated from the World oceans and had subsided possibly up to some 700 m below their level at the beginning of Upper Rotliegend sedimentation. After catastrophic flooding of this paleo-topographic depression at the beginning of the Zechstein, changing sea level, sedimentation and subsidence rates remained essentially in balance. Although the effects of Triassic rifting overprinted parts of the Southern Permian Basin, its overall subsidence pattern persisted well into the Jurassic. In contrast to the remainder of the Southern Permian Basin, Permian and Triassic crustal extension contributed significantly to the subsidence of the Polish Trough.

Role of pre-rift rheology in kinematics of extensional basin formation: constraints from thermomechanical models of Mediterranean and intracratonic basins

The role of pre-rift rheology on the kinematics of extensional basin formation is examined. Constraints obtained on the effective elastic thickness and level of necking, inferred from forward modelling of a number of Alpine/Mediterranean basins (including the Gulf of Lion margin, the Valencia Trough, the Tyrrhenian Sea and the Pannonian Basin), are interpreted in terms of the pre-rift rheology of the lithosphere underlying these basins. The Gulf of Lion/Tyrrhenian Sea basins and the Pannonian Basin appear to be end-members in terms of the inferred depth levels of necking. The models support the existence of spatial variations in crustal and lithospheric strength, as inferred from previous rheological modelling for other segments of the European lithosphere, and provide constraints on the ratio of crustal and subcrustal strength during extension. The results of these studies are compared with predictions on the kinematics of extension for a number of intracratonic basins, including the Black Sea basins, the Transantarctic Mountains/Ross Sea and Saudi Arabian Red Sea margins, and the Baikal and East African rifts. The kinematics of extension appears to be largely controlled by the (transient) thermal regime of the pre-rift lithosphere and the crustal thickness distribution. These usually result from orogenic processes operating on the lithosphere before extensional basin formation. Predictions are made for the level of external forces required to initiate rifting in intracratonic and Alpine/Mediterranean settings. The models also shed light on the relative parts played by far-field versus near-field stresses and inferred variations in strain rate during the evolution of these basins.

3D Flexure and intraplate compression in the North Sea Basin

We apply a recently developed 3D flexure model incorporating lateral variations in flexural rigidity (EET) and necking depth (zn) to study the 3D effects of intraplate stresses on Quaternary accelerated subsidence and uplift in the North Sea Basin and adjacent areas. In the model approach lateral variations and magnitude of predicted Quaternary vertical motions are largely dependent on the pre-existing Late Tertiary flexural state of the area and the relative change in magnitude and orientation of intraplate stresses in the Quaternary. The Late Tertiary deflections are reconstructed by incorporating lithospheric stretching values calculated from the Mesozoic subsidence record, and by adopting admissible variations in necking depth (z,) and flexural rigidity (EET) from earlier work. The 3D model results for the North Sea and adjacent areas indicate that an increase of compressive intraplate forces with a magnitude of about 2.25 × 10 ~2 N/m can predict accelerated subsidence values up to 700 m, largely in agreement with observed patterns of Quaternary isopach values corrected for effects of shallowing waterdepth. The magnitude of the intraplate forces is 2 to 3 times lower than predicted by earlier 2D studies. It is believed that the 2D model results actually overestimate required stress levels, since they do not take into account the effect of out-of-plane stresses.The relative increase in compressive forces is in agreement with observed compressive stresses and the magnitude corresponds to characteristic values for plate boundary forces. The adopted values for Zn and EET do not show a clear relation with the preceding basin history.

Eastern Alpine tectono-metamorphic evolution: Constraints from two-dimensional P-T-t modeling

We use two-dimensional (2-D) P-T-t modeling to constrain the thermal and rheological aspects of different scenarios for the late Mesozoic and Cenozoic tectonic evolution of the Eastern Alps, inferred from excellent data sets from the Tauern Window (TW). Models invoking subduction of the South Penninic (SP) oceanic lithosphere during thrusting and subsequent erosion of the Austro-Alpine (AA) upper plate nappe stack are inconsistent with the observed thermal evolution within the AA and Penninic units. In these models, predictions for the AA peak thermal conditions are lower than observed. After exhumation and cooling to midcrustal levels and subduction of the continental Middle Penninic (MP) block, the AA undergoes a phase of renewed heating to almost the previous peak temperatures. Simultaneously, the Penninic units experience a phase of heating upon subduction, followed by cooling after onset of subduction of the North Penninic (NP) basin. The model predictions are inconsistent with the observed nearly isothermal uplift path of the SP after subduction and cannot explain observed inverted metamorphic peak conditions in the deeper AA (amphibolite facies) down to the higher Penninic unit (greenschist facies). A model with the beginning of subduction of the SP occurring after crustal thickening of the AA and subsequent return to normal crustal thicknesses is compatible with the P-T-t data. In this model, peak temperature conditions are higher in the AA, followed by a phase of strong cooling in the AA upper plate with the onset of underthrusting. This model also explains successfully nearly isothermal exhumation of the MP and inverted metamorphic peak conditions in the deeper AA. Material accreted to the hanging wall from the oceanic crust (SP) experiences a phase of cooling during ongoing subduction of oceanic lithosphere and begins to heat up to its thermal climax after the subduction of trailing continental lithosphere. The subsequent PT path of the Penninic units strongly depends on the timing and rates of underthrusting by the foreland. Observed PT paths in the MP within the TW require continuous subduction of the NP and the trailing European foreland under the exhuming MP block. Documented rapid cooling in the final uplift phase of the Penninic units in the TW requires exhumation rates up to approximately 4 mm/yr. Predictions of slightly elevated present-day geothermal gradients in the TW area are consistent with available heat flow data. As a result of Mesozoic rifting followed by late Mesozoic crustal thickening of the AA, paleorheological reconstructions are characterized by a contrast between relatively strong oceanic lithosphere and adjacent weak continental lithosphere. Predicted decoupling of weak continental and oceanic lithosphere during subsequent subduction of the Penninic units can explain observed Late Cretaceous crustal extension in the AA units in terms of gravitational spreading. Ongoing subduction leads to an overall strength increase due to underthrusting of cool oceanic lithosphere, whereas subduction of continental lithosphere causes a strength decrease in the upper levels of the lithosphere. Continuous crustal thickening and relaxation of the depressed isotherms reduce the strength of the lower lithospheric mantle beneath the central orogen, further enhanced by rapid late-stage uplift. Predictions for the present-day rheological structure of the Eastern Alps support the existence of a strong upper crustal layer, two wedge-shaped strong upper mantle layers to the north and the south of the orogen, and a weak upper mantle underlying the central orogen.

Post-Variscan evolution of the lithosphere in the Rhine Graben area: constraints from subsidence modelling.

Abstract In the area of the Cenozoic Rhine rift system, crustal and lithospheric thicknesses range between 24 and 35 km, and 60 and 120 km, respectively. This rift system transects the deeply truncated Variscan Orogen and superimposed Permo-Carboniferous wrench-induced troughs, and Late Permian and Mesozoic thermal sag basins. At the time of its Westphalian consolidation, the Variscan Orogen was probably characterized by 45–60 km deep-crustal roots that were associated with its Rheno-Hercynian-Saxo-Thuringian, Saxo-Thuringian-Bohemian and Bohemian-Moldanubian sutures, all of which are transected by the Cenozoic Rhine rift system. During the Stephanian-Early Permian wrench-induced disruption of the Variscan Orogen, subducted lithospheric slabs were detached causing upwelling of hot mantle material. During the resulting thermal surge, partial delamination and/or thermal thinning of the continental mantle-lithosphere induced regional uplift. At the same time the Variscan orogenic roots were destroyed and crustal thicknesses reduced to 28–35 km in response to the combined effects of mantle-derived melts interacting with the lower crust, regional erosional unroofing of the crust and, on a more local scale, by its mechanical stretching. Towards the end of the Early Permian, the potential temperature of the asthenosphere returned to ambient levels. With this, regional, long-term thermal subsidence of the lithosphere commenced, controlling the development of a new system of Late Permian and Mesozoic thermal sag basins. However, the evolution of these basins was repeatedly overprinted by minor short-term subsidence accelerations that reflect the build-up of far-field stresses related to rifting in the Tethyan and Atlantic domains. Comparison of observed and modelled subsidence curves suggests that in the area of the Rhine rift system the lithosphere had equilibrated with the asthenosphere at the end of the Cretaceous at depths of 100–120 km, before it became thermally destabilized again by Cenozoic rifting and plume-related magmatism. Modelled subsidence curves indicate that by the end of Early Permian times the thermal thickness of the remnant mantle-lithosphere ranged between 10 and 50 km in areas that were later incorporated into Mesozoic thermal sag basins; this corresponds to mid-Permian thermal lithosphere thicknesses of 40–80 km.

Temporal and spatial variations in tectonic subsidence in the Iberian Basin (eastern Spain): inferences from automated forward modelling of high-resolution stratigraphy (Permian–Mesozoic)

By subsidence analysis on eighteen surface sections and 6 wells, which cover large part of the Iberian Basin (E Spain) and which are marked by high-resolution stratigraphy of the Permian, Triassic, Jurassic and Cretaceous, we quantify the complex Permian and Mesozoic tectonic subsidence history of the basin. Backstripping analysis of the available high resolution and high surface density of the database allows to quantify spatial and temporal patterns of tectonically driven subsidence to a much higher degree than previous studies. The sections and wells have also been forward modelled with a new ‘automated’ modelling technique, with unlimited number of stretching phases, in order to quantify variations in timing and magnitude of rifting. It is demonstrated that the tectonic subsidence history in the Iberian Basin is characterized by pulsating periods of stretching intermitted by periods of relative tectonic quiescence and thermal subsidence. The number of stretching phases appears to be much larger than found by earlier studies, showing a close match with stretching phases found in other parts of the Iberian Peninsula and allowing a clear correlation with discrete phases in the opening of the Tethys and Atlantic.

Tectonic variation in the Dniepr-Donets Basin from automated modelling of backstripped subsidence curves

Abstract The multiphase rift evolution of the Dniepr-Donets (DD) Basin is analysed by 1D backstripping and tectonic modelling of 66 stratigraphic sections, mostly from well data, distributed throughout the basin. The backstripping results illustrate strong temporal and lateral variations in tectonic subsidence. It is minor during an ‘inceptive’ pre-rift phase (Givetian-early Frasnian; 380.8-370 Ma) and does not show clear spatial correlations within the DD Basin framework. The main rift phase (Frasnian-Famennian; 370-362.5 Ma) corresponds to an acceleration in subsidence rates with tectonic subsidence up to 1500 m along the basin axis, displaying a slight increase from the northwest to the southeast. Strong lateral variations perpendicular to the basin axis reflect the effects of major basin border faults. A rift phase in the Visean (345-340 Ma) is characterised by relatively minor subsidence (up to about 500 m, except for some wells in the southeast), following a stage of extremely low rates of deposition in the earliest Carboniferous. The end of this rift phase can be taken to mark the onset of convex subsidence patterns in the later Carboniferous and Permian, typical in shape of post-rift thermal subsidence, but significantly increasing in magnitude from northwest to southeast in the basin. This trend shows no one-to-one relationship with Late Devonian syn-rift subsidence. The tectonic subsidence curves have been modelled using lithospheric stretching assumptions allowing different stretching factors for the crustal and sub-crustal lithosphere and the incorporation of finite and multiple stretching phases. A numerical technique was developed that automatically finds best fit stretching parameters for part or all of the tectonic subsidence data, given the specification of onset and duration of the various rifting events. Maps of the 1D modelling results display intrabasinal variations in terms of timing and magnitude of rifting. The results indicate that the observed spatial and temporal variations of tectonic subsidence in the DD Basin, specifically the increasing ratio of Carboniferous to Late Devonian subsidence from northwest to southeast, cannot be explained by either a uniform or depth-dependent stretching model adopting only a single Frasnian-Famennian rift phase. A two-phase model, in which a significant part of Carboniferous tectonic subsidence is related to Visean rift activity, strongly dominated by sub-crustal lithospheric attenuation, provides a satisfactory fit to the data although the tectonic implications of such a model are not easily reconcilable by independent constraints. A semi-quantitative investigation of the possible role of withdrawal of underlying Devonian salt during the Carboniferous to produce additional post-Devonian rift phase sediment accommodation space suggests that the effects of such a mechanism could be significant but that they are unlikely to preclude the necessity of the Visean rifting event to explain the Carboniferous basin evolution.

Pressure-temperature-time evolution of the high-pressure, metamorphic complex of Sifnos, Greece

Model studies of continental collision tectonics and pressure-temperature-time ( P - T - t ) predictions quantitatively demonstrate the key role of synsubduction uplift in the preservation of blueschist- and eclogite-facies metamorphic rocks. Rheological stratification of the subducting lithosphere allows development of detachment faults at the compositional boundary between the upper and lower crust. Material of supracrustal continental affinity is subducted to depths in excess of 50 km and then delaminated from the downgoing slab. Subduction continues while the detached domain is uplifted independently from the newly active hanging walls and footwalls. Two-dimensional thermal calculations indicate that the nearby active Subduction provides the cooling mechanism required to preserve blueschists and eclogites, whereas the differential motions of the independently uplifted domains can explain the observed heterogeneity in P - T - t pathways, with some domains displaying a greenschist overprint.